The new study looks at the Red List, a catalogue of extinct and threatened species drawn up by the International Union for Conservation of Nature (IUCN). For species that are completely extinct or extinct in the wild, those who draw up the list identify one or more contributing factors.

Tim Blackburn at University College London and his colleagues compiled data from the Red List on 247 species of plant, amphibian, reptile, bird and mammal that have disappeared since 1500. They found that invasive species are the second most common threat associated with the losses, behind hunting, fishing or harvesting. For amphibians, mammals and reptiles, invasive species were the number one threat.

Cats, rats and goats

A relatively small number of species are blamed: cats, rats and goats are among the most common offenders, along with microorganisms like the amphibian-killing chytrid fungus and the avian malaria parasite.

But other ecologists say the threat posed by non-native species has been overstated. Mark Davis from Macalester College in Saint Paul, Minnesota, points out that 86 per cent of the extinctions ascribed to invasive species occurred on islands, where endemic species have small populations and are poorly adapted for predators or environmental changes. On continents, it’s a different story.

“The data shows that extinctions on continents are seldom caused by introduced species,” he says. “They are not a primary driver on a global scale.”

Blackburn agrees that alien species are not as big an issue on continents as other factors such as habitat destruction.

“Nevertheless, I don’t think we should ignore the impact of aliens on continents,” he says. “There are some very high profile examples of where aliens have had big impacts on continents, particularly in Australia.”

Examples include introduced European red foxes that had negative impacts on many species, and were implicated in the extinction of the desert rat kangaroo, and feral cats are said to have caused the extinction of several ground-dwelling birds.

Jessica Gurevitch at Stony Brook University in New York says the Red List data cannot tell if an invasive species causes an extinction or takes advantage of native species that are already in decline. “It’s impossible to determine what the causes of past extinctions have been.”

Habitat loss and climate change

The WWF’s Living Planet Index ascribes invasive species as the primary threat for only 5 per cent of the vertebrate species listed. Exploitation, habitat degradation and change, habitat loss and climate change are all rated as bigger threats.

Blackburn refutes the claims made by Davis and others that non-native species have been unfairly vilified.

“This alien species denialism is a bit nonsensical,” he says. “Clearly there are alien and invasive species that cause problems, and there are some for which there doesn’t seem to be much in the way of evidence they have issues. But to say we shouldn’t worry about aliens because some aliens don’t have problems is equivalent to saying we shouldn’t worry about bacteria because some bacteria don’t cause diseases.”